Thermal ink jet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and by U.S. Pat. Nos. 4,490,728 and 4,313,684. Thermal ink jet printers produce high quality print, are compact and portable, and print quickly but quietly because only ink strikes the paper. The typical thermal ink jet printhead uses liquid ink (i.e., colorants dissolved or dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal ink jet firing resistor, a small portion of the ink adjacent to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
High performance, high speed thermal ink jet printheads generate large quantities of heat. When printing at maximum output (i.e., in "black-out" mode in which the printhead completely covers the page with ink), the rate of heat generation by thermal ink jet printheads is comparable to that of small soldering irons. Some of the heat is transferred directly to the ink in the firing chamber, but the printhead substrate absorbs the, balance of this energy which will be called the "residual heat". (The rate of residual heat generation will also be referred to as the "residual power".) The residual heat can raise the overall printhead temperature to values that cause the printhead to malfunction. Under extreme circumstances, the ink will boil with severe consequences.
Existing printheads require air Cooling in steady-state operation. Heat sinks are used to reduce the thermal resistance between the printhead and the surrounding air, thus enabling rejection of the residual heat at an acceptable printhead temperature. Heat sinks have high thermal conductivity and large surface area. They may be special-purpose devices (e.g., metal fins) or devices with a different primary function (e.g., a chassis). Often, an integral ("on-board") ink reservoir serves as a heat sink for the printhead.
Here, the term "heat sink" refers to any device used to reduce the steady-state thermal resistance between the printhead and the surrounding air. (It is not to be confused with purely capacitive devices which function only in a transient mode.) This thermal resistance is the sum of two components: (1) the thermal resistance between the printhead and the external surface that transfers the heat to the air and (2) the convective thermal resistance between the external heat transfer surface and the surrounding air. (For the heat sink to be effective, this sum must be substantially less than the convective thermal resistance between the printhead alone and the surrounding air.) The first resistance component depends on the internal constitution of the heat sink and various schemes are used to reduce its value. These include the use of high conductivity materials, short heat flow paths, thermal conductors of large cross-sectional area, fins extending into the integral ink reservoir, and/or a miniature pump to circulate ink from the integral reservoir past the printhead and back to the reservoir. The second resistance component is inversely proportional to the area of the external heat transfer surface. Generally, a heat sink is large if its total thermal resistance is low.
A disadvantage of heat sinks is that their steady-state heat transfer rate is proportional to the printhead temperature and this causes the printhead temperature to vary strongly with the firing rate. When the firing rate increases (decreases), the residual power increases (decreases) and the printhead temperature increases (decreases) until the rate of heat rejection is equal to the residual power. For each firing rate there is a different equilibrium temperature at which there is no net flow of heat into (out of the printhead substrate. Since the firing rate varies widely during normal printer operation, large printhead temperature variations are expected.
Fluctuations in the printhead temperature produce variations in the size of the ejected drops because two properties that affect the drop size vary with printhead temperature: the viscosity of the ink and the amount of ink vaporized by the firing resistor. Drop volume increases with temperature and excessive temperatures will cause undesirable large drops and unwanted secondary drops. When printing in a single color (e.g., black), the darkness of the print varies with the drop size. In color printing, the printed color depends on the size of each of the primary color drops that create it. Thus, dependence of printhead temperature on firing rate can severely degrade print uniformity and quality. Also, a wide operating temperature range generally necessitates the use of an increased pulse energy to ensure proper ejection of cold and viscous ink and thus increases power consumption and decreases the life and reliability of the firing resistors.
The printhead temperature can be stabilized by adding heat to the substrate to maintain it at a temperature that is equal to the equilibrium temperature for its highest firing rate. In this case, a heat sink will require that, under all operating conditions, the sum of the residual power and the additional power be equal to the residual power at the maximum firing rate. This excessive power consumption is especially disadvantageous in battery operated printers.
Also, heat sinks have the disadvantages of adding significant thermal capacitance, mass, and volume to the printhead. The additional thermal capacitance increases the warm-up time of the printhead during which the print quality is degraded for the reasons discussed above. The mass of a heat sink large enough to cool a high-speed, high-performance printhead would impair the high speed capabilities of such a printhead by limiting its traverse accelerations. And the large volume of a heat sink is obviously undesirable for a moving part in a compact device. A heat sink consisting of the ink reservoir has the additional disadvantage of subjecting the ink supply to elevated temperatures for extended periods of time, thus promoting thermal degradation of the ink.